Much work has been undertaken in recent years to develop polymers for foodstuff packaging materials which have a high degree of impermeability to passage of oxygen, carbon dioxide and water vapor. Polymeric materials for the most part have serious deficiencies where barrier properties are needed to prevent passage of oxygen, carbon dioxide and/or water vapor in comparison to a packaging material made from glass or metal. Illustratively, in the field of carbonated beverage bottles for packaging soda or beer, the ability of packaging material to minimize the migration of carbon dioxide through the walls of the bottle is absolutely essential if the package is to have the requisite shelf life.
It has been long recognized that acrylonitrile copolymers have desirable barrier properties for preventing passage of oxygen, carbon dioxide and/or water vapor. However, copolymers with high acrylonitrile content may possess undesirable processing characteristics. As the acrylonitrile content in the polymer increases to unduly high levels, the polymer becomes intractable under conventional processing conditions. The use of polymerizable monomers in conjunction with the acrylonitrile component to form a copolymer has enabled the use of conventional processing conditions in fabrication of packaging materials; for example, bottles, sheets, etc.
A teaching directed to formation of acrylonitrile copolymers is set forth in Trementozzi, U.S. Pat. No. 3,451,538 issued June 24, 1969 with utility disclosed in packaging food and medicine. Another prior art teaching is set forth in Howe, U.S. Pat. No. 3,819,762 issued June 25, 1974 whereby acrylonitrile copolymers have desirable physical and chemical properties due to uniformity in the polymer chain obtained by maintaining during polymerization a controlled ratio of acrylonitrile and a second monomer reactant. Uniformity in the polymer chain is considered to account for superior properties in the polymer.
In both of U.S. Pat. Nos. 3,451,538 and 3,819,762 described above, batch polymerization is employed to obtain the acrylonitrile polymer. As a practical matter in commercial manufacture of large quantities of polymers, continuous polymerization is desirable to reduce the cost.
Continuous polymerization techniques are well known in the art both in one or more reactor modes. A teaching disclosing continuous polymerization of acrylonitrile and styrene in a two reactor system is set forth in Murray U.S. Pat. No. 3,547,857 patented Dec. 15, 1970. This patent deals with formation of copolymers with relatively small amounts of acrylonitrile polymerized units. An earlier prior art teaching directed to a two-reactor system for emulsion polymerization is found in Carroll et al., U.S. Pat. No. 2,872,438 issued Feb. 3, 1959. Other disclosures employing the concept of polymerization using seed particles and/or two or more reactor systems include U.S. Pat. No. 2,475,016 and British Specification Nos. 1,168,760 and 1,324,955.